For safety reasons there is a clear desire to detect anomalies in the wheelset of a rail vehicle. Failure of even a single wheel of a train could potentially lead to derailment with potentially drastic consequences.
One such wheel defect which it is wished to detect is a wheel flat. A wheel flat is a fault in the shape of the wheel, for example an area of the wheel which is deformed from the normal generally circular shape. A wheel flat may for instance comprise a relatively flat spot on the wheel. Wheel flats can be caused, for example, by skidding of the train wheelset along a rail. This may occur for instance under some braking conditions, for example heavy braking or if the train brakes are not applied correctly. Slippery rail or slip-and-slide conditions can also cause a train wheel to start a skid which can cause problem if the skid extends beyond the slippery section of the rail. The wheel shape can be deformed as the wheel is dragged along the rail.
A wheel developing a wheel flat can be problematic in various ways. The process of generating the wheel flat may have damaged the wheelset, for instance due to the heat generated during the wheel skid. Additionally the area with the wheel flat will tend to impact on the rail during the wheel rotation. The repeated impact on the wheel can lead to further damage of the wheel and/or possibly the associated bearing assembly. In addition the wheel flat may cause damage to rail track itself. In extreme cases this can lead to the wheel or the rail track failing catastrophically. The wheel flat may also deform the wheel altering the clearance between the wheel and the brake block which may cause problems.
There is therefore a clear desire to detect wheel flats so that preventative maintenance can be applied. The most basic method of wheel flat detection is visual inspection of the train wheels by maintenance personnel. However such a method is time consuming as each wheel must be checked individually and typically the train must be moved between two or more inspections of the wheel or perhaps moving continuously at a slow speed so that the whole perimeter of each wheel can be inspected.
Some methods of wheel flat detection rely on detecting the conditions likely to lead to a wheel flat, e.g. skidding of a train wheel. If the train or a particular wheel is determined to have skidded this may trigger an alert and prompt an appropriate inspection.
It has also been proposed to use acoustic monitoring to provide detection of wheel flats. As mentioned above, as a wheel with a wheel flat rotates the wheel flat will impact on the rail, which can produce an impact noise. As the train moves the repeated impacts from a wheel flat will therefore typically lead to a repetitive noise due to the repeated impacts. This repetitive impact noise can be used as an acoustic signature indicative of a wheel flat. It has therefore been proposed to place an acoustic transducer such as a microphone by the side of a rail track to listen to the sounds generated by a train as it passes. If an acoustic signal with a significant periodic component were to be detected this could be used as indication of a possible wheel flat.
Acoustic monitoring for wheel flat detection offers several benefits. A single trackside sensor can provide automatic monitoring of all trains that pass that sensor and doesn't require any sensing equipment on the train itself.